reflex -precise motions in response to afferent stimuli -mediated at all levels of CNS -rapid initiation -many elicited even during unconsciousness

myotatic reflex

-stretch reflex-appearance: contraction (shortening) of the stretched muscle-purpose: protect muscle from tearing from stretch-characteristics: -initiated by muscle spindle -monosynaptic, segmental reflex--afferent info comes into the same segment of the SC that the motor will leave

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muscle spindle

-myotatic reflex-extrafusal fibers--fibers you see when looking at a M and they are the fibers that do the work-intrafusal fibers -parallel to extrafusal fibers -have an afferent sensory component in the center and an efferent motor component at the periphery/ends

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sensory portion of the intrafusal M fibers of the muscle spindle

-sensory portion: -not contractile -portion sensitive to length -actually 2 different afferents 1. nuclear bag fiber--nuclei are in the center in a swelling 2. nuclear chain fiber--nuclei are lined up in a row

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primary afferent in the sensory portion of the intrafusal M fibers of M spindle

type 1 A fiber -innervates both the nuclear bag and nuclear chain fiber -large, myelinated 1a fiber -sensitive to both: -length of M -how fast the length is changing -fires a few APs per sec at rest, but fires inc APs when the muscle spindle is stretched

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secondary afferent in the sensory portion of the intrafusal M fibers of M spindle

type 2 fiber -slightly smaller than type 1A fiber -innervates only the nuclear chain fiber -sensitive to only the length of the M--tells us what the length of the M is RIGHT NOW, and doesn't give any info o if the M is changing -less myelin

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motor portion of the intrafusal M fibers of the muscle spindle

-same as skeletal M-innervated by a gamma motoneuron--this neuron goes thru NMJ and releases ACh-control the length of the sensory portion

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how does the contraction of the intrafusal sensory and motor fibers

-by contracting the intrafusal Ms, we stretch the sensory portion, this renders the sensory portion more sensitive to a superimposed stretch -the motor portion on each end is attached to ligaments/tendons that the M is attached to and as we contract the motor part the sarcomeres move closer together, and we stretch the sensory portion nuclear bag/chain-gamma motoneurons control the sensitivity

how does the myotatic reflex work for the agonistic M

-1a afferent neuron comes into the SC and releases EAA onto the alpha motor neuron -motoneuron is excited by the activation of the 1a afferent-->contraction relieves the stretch, returning the 1a discharge rate back to normal

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how does the myotatic reflex work for the antagonistic M

-when elicit a stretch reflex on the agonistic M and contract the M, the antagonistic M is also stretched but don't get another stretch reflex-when 1a afferent comes in, it sends a branch to an interneuron that inhibits the alpha motor neuron to the antagonist M which allow the antagonist M to relax and allow the original M to experience the reflex--RECIPROCAL INHIBITION -anytime you contract one M, their antagonist M must relax -there is a decrease in the activity of the alpha motoneuron innervating the antagonist M allowing it to RELAX and LENGTHEN

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golgi tendon reflex

-also called inverse myotatic reflex and autogenic inhibition-appearance: sudden (abrupt) relaxation of a contracted M-purpose: protect M from damage due to excessive force-chracteristics: -initiated by the golgi tendon organ -polysynaptic, segmental reflex--more than one synapse, but afferent and efferent come out the same segment

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golgi tendon organ

how does the golgi tendon reflex work

-1b afferent come from the golgi tendon organ and into the SC and is excitatory to so releases EAA to the interneuron in the SC -activated by the EAA and then this interneuron releases an inhibitory NT (glycine) which inhibits the alpha motor neuron of the contraction M -so this alpha motor neuron gets input from brain to contract and inhibitory input form the SC to relax--so when inhibition finally comes thru, it stops contraction of the alpha motor neuron-these reflexes allow us to avoid going to the brain to get the contraction to stop

-motoneuron is inhibited by the activation of the spinal interneuron-->abrupt relaxation of the M occurs, returning the golgi tendon organ discharge rate to normal

modification of reflexes

-occurs all the time--healthy-a lot of the time, the influence for modification is inhibitory

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spinal shock

-occurs from spinal transection--from hyper polarization of spinal neurons due to loss of excitatory input form cortex-reflexes that are below the level of the transection, the reflex fails to occur even though the wiring is intact -occurs right after the spinal cord is transected but usually goes away and reflexes can return, except in humans, the Ms often atrophy w/in the year and the reflex return is never noticed-reversible -recovery results from axonal sprouting below the level of the transection--occurs due to neurons need for input b/c without input they will die -recovery also occurs due to expression of R phenotypes that are self-activating

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decerebrate posturing

results from loss of all structures rostral to the pons-involves the whole body-spinal reflexes are intact and hyperactive -experience extension in all Ms, so rigid--happens instantly -usually bilateral

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rigidity vs. spasticity

rigidity: -resists motion in all directions -results from maintained M contraction; loss of cortical influence that inhibits a medullary input to the alpha motoneurons -continual activation of alpha motoneurons -not due to reflexes -brainstem is activating alpha motoneurons

spasticity: -resists motion in a given direction--so when move arm, the arm will spring back -myotatic reflex is hyperactive due to inc gamma motoneuron firing -continual activation of gamma motoneurons -contracts intrafusal M -lengthens nuclear bag/chain fibers -brainstem -cause: damage to cortex that abolishes activation of the brainstem inhibitory region

decorticate posturing

-rigidity -flexion of the upper limb joints -extension of the lower limbs -dependent on head position -head motion in one direction will flex the contralateral arm and extend the ipsilateral arm-changes as the head is moved passively-happens often b/c it happens in pts that experience strokes in the vicinity of the internal capsule -cause: loss of cortical inputs due to lesion of internal capsule

hopping rxn

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what innervates the intrafusal M fibers?

gamma motoneuron

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in order to save time, Dr. K tries to lift 47 cases of Pepsi into car in one lift. She is not up the challenge and a reflex is elicited, resulting in complete relaxation of her Ms. Activation of what produces this?

1b fiber b/c this is involved in the golgi tendon reflex which protects M from damage due to excessive force and causes abrupt relaxation of a contracted M

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in order to save time, Dr. K tries to lift 47 cases of Pepsi into car in one lift. She is not up the challenge and a reflex is elicited, resulting in complete relaxation of her Ms. How many synapses in the SC are required for reflex?

2 1b afferent from golgi tendon--interneuron--alpha motorneuron

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in order to save time, Dr. K tries to lift 47 cases of Pepsi into car in one lift. She is not up the challenge and a reflex is elicited, resulting in complete relaxation of her Ms. what R is most likely on the alpha motoneuron in this reflex?